Mode-mismatched Configuration Research Articles

Comparison between low optical absorption models for pump-probe transient thermal lens spectroscopy: a theoretical and experimental study

We compare the most widely used low optical absorption models based on the Fresnel diffraction theory for the description of the time-resolved thermal lens signal in a dual-beam pump-probe mode-mismatched configuration. In this work, we will name one of them as the numerical mode-mismatched model. The others are two simplified approximations of it that will be referred to as the analytical mode-mismatched and the reduced mode-mismatched (RMM) model. The limits of application of these approximations are discussed based on computational calculations and experimental measurements performed under conditions that neglect contributions to the thermal lens signal generation of mechanisms different from optical absorption, heat diffusion and the propagation of the excitation and probe laser beams. It is shown that for fractional changes of the probe beam greater than about 15% these approximations must be used carefully, particularly the RMM model.

Thermal lens spectroscopy dosimetry at high doses using a commercial transparent glass

A dual-beam mode-mismatched configuration of the thermal lens spectroscopy technique was applied to characterize commercial transparent glass irradiated with high doses of beta radiation (90Sr+90Y). Thermo-optical properties of the glass were studied after irradiation within an approximate dose range of 1.103–7.103 Gy. The glass family and structural group type as well as its amorphous nature were confirmed by Raman spectroscopy and X-ray diffraction. The spectral absorption in UV–Vis range of the samples revealed changes due to radiation exposure. The parameters of thermal diffusivity and phase difference were investigated as a function of the radiation doses, using a CW diode laser (488 nm and 40.0 mW) for excitation and a HeNe laser (632.8 nm) as a probe beam. The theta phase difference and the steady-state thermal lens signal presented a linear response in the dose range studied. Thermal lens spectroscopy was found to be a relatively good candidate as a dosimetry technique that can be useful for investigation of dosimetry characteristics in new glasses.

Determination of the biodiesel content in diesel/biodiesel blends by using the near-near-infrared thermal lens spectroscopy

A dual-beam mode-mismatched configuration of the Thermal Lens (TL) technique was applied to characterize biodiesel, diesel and diesel/biodiesel blends. The excitation was performed using a tunable laser in the near-near infrared region in order to assess overtones and combination bands of the vibration modes. The thermo-optical properties of the fuels were also determined. The results showed that the biodiesel contents in diesel/biodiesel blends can be monitored with this technique, by excitation the third overtone of the C–H (4ν(C–H)) bond stretching vibration at 916 nm. Both the amplitude of the photothermal signal and thermal diffusivity are important parameters to quantify the biodiesel content in diesel/biodiesel blends.

Determination of Fe(II) by Optimized Thermal Lens Microscope

A simple and accurate method for the determination of Fe(II) species in liquid solutions was developed. The measurement principle was realized by the photothermal effect and performed by the use of a thermal lens microscope (TLM) in its optimized, mode-mismatched configuration with a counter-propagating probe beam (PB) and an excitation beam (EB). Additionally, due to the combination of the collimated PB and tightly focused EB measurement, the detection limit was significantly decreased. Our preliminary results demonstrated that a TLM enables the detection of iron ions at the levels of $$\upmu \mathrm{g}{\cdot }\hbox {L}^{-1}$$ , which makes this technique an alternative to traditional spectroscopic methods as a promising detector for the ultrasensitive determination of different compounds in liquid solutions.

Mode-mismatched confocal thermal-lens microscope with collimated probe beam.

We report a thermal lens microscope (TLM) based on an optimized mode-mismatched configuration. It takes advantage of the coaxial counter propagating tightly focused excitation and collimated probe beams, instead of both focused at the sample, as it is in currently known TLM setups. A simple mathematical model that takes into account the main features of the instrument is presented. The confocal detection scheme and the introduction of highly collimated probe beam allow enhancing the versatility, limit of detection (LOD), and sensitivity of the instrument. The theory is experimentally verified measuring ethanol's absorption coefficient at 532.8 nm. Additionally, the presented technique is applied for detection of ultra-trace amounts of Cr(III) in liquid solution. The achieved LOD is 1.3 ppb, which represents 20-fold enhancement compared to transmission mode spectrometric techniques and a 7.5-fold improvement compared to previously reported methods for Cr(III) based on thermal lens effect.

Development of Reflection Objective-Employed Collinear Mode-mismatched Thermal Lens Microscope

A reflection objective-employed collinear thermal lens microscope (roco-TLM) is developed in which a reflection objective lens is used for collinearly focusing, in a mode-mismatched configuration, both an excitation laser beam at 260 nm and a probe laser beam at 632 nm. With the microscope, unsymmetrical distortions of a thermal lens image obtained with a reflection objective were reduced. The cause of the diffraction pattern in a thermal lens image observed with the roco-TLM is discussed as well.

Development of Reflection Objective-Employed Collinear Mode-mismatched Thermal Lens Microscope

A reflection objective-employed collinear thermal lens microscope (roco-TLM) is developed in which a reflection objective lens is used for collinearly focusing, in a mode-mismatched configuration, both an excitation laser beam at 260 nm and a probe laser beam at 632 nm. With the microscope, unsymmetrical distortions of a thermal lens image obtained with a reflection objective were reduced. The cause of the diffraction pattern in a thermal lens image observed with the roco-TLM is discussed as well.

Thermal lens spectrometry in pyroelectric lithium niobate crystals

In this work, the light-induced lens effect due to thermal and/or photorefractive processes was studied in pyroelectric (undoped and Fe2+-doped) lithium niobate crystals (LiNbO3) using thermal lens spectrometry with a two-beam (pump–probe) mode-mismatched configuration. The measurements were carried out at two pump beam wavelengths (514.5 and 750 nm) to establish a full understanding of the present effects in this material (thermal and/or photorefractive). We present an easy-to-implement method to determine quantitative values of the pyroelectric coefficient (dP s/dT), its contribution to the thermal effect and other thermo-optical parameters like thermal diffusivity (D), thermal conductivity (K) and temperature coefficient of the optical path length change (ds/dT). These measurements were performed in LiNbO3 and LiNbO3:Fe (0.1 ppm Fe2+) crystals with c axis along the direction of laser propagation.

Angular dependence of the thermal-lens effect on LiSrAlF_6 and LiSrGaF_6 single crystals

We apply thermal-lens (TL) spectrometry to measure the angular dependence of the TL effect on colquiriite single crystals. The experiments were performed with LiSrAlF(6) and LiSrGaF(6) using a two-beam mode-mismatched configuration. The results show that it is possible to minimize the TL effect by selecting the appropriate crystal orientation. Our data also show that the anisotropy of the linear thermal expansion coefficient drives the amplitude of the TL effect, including the inversion from focusing to defocusing as the crystal orientation angle tends to the c-axis direction. The results may be useful for those working to develop a high-power laser using LiSrAlF(6)(:Cr) and LiSrGaF(6)(:Cr) single crystals, allowing for optimization of the designed laser cavity.

Photothermal Lensing Signal Enhancement by the Transient Absorption of Photoexcited States in Liquid Solutions

A two-color double-pulse excited photothermal lensing measurement was performed to investigate a signal enhancement owing to transient absorption by the excited states of solute molecules in liquid solutions. A colinear, mode-mismatched configuration with a continuous-wave probe laser at 633 nm was adapted with a focused pump pulse (6.9 ns, 355 nm) and a sequentially incident softly focused second-pump pulse (7.2 ns, 532 nm). The signal-intensity dependence on the energies of the pump and the second-pump pulses was measured for 1.0 µM solutions of 2,2′-p-phenylenebis(5-phenyloxazole) in 1-butanol and anthracene in ethanol. A signal enhancement was observed for both solutions. The signal intensity under certain conditions was one order higher than that expected from ground-state absorption. A discussion was made on determination of the absorption coefficient of the transient species and on the absorption saturation of the ground and excited states.

Probe-beam diffraction in a pulsed top-hat beam thermal lens with a mode-mismatched configuration

The Fresnel diffraction integral is used directly to describe the thermal lens (TL) effect with a mode-mismatched collinear configuration. The TL amplitudes obtained with Gaussian, Airy, and top-hat beam excitations are computed and compared. Numerical results for beam geometries optimized for both near- and far-field detection schemes are presented, and the analytical results developed by Bialkowski and Chartier [Appl. Opt. 36, 6711 (1997)] for a Gaussian beam TL effect are summarized in simplified form. Both the numerical and the analytical results demonstrate that, under a beam geometry optimized for either near- or far-field detection, the Gaussian beam TL experiment has approximately the same maximum signal amplitude as does the photothermal-interference scheme. A comparison between the optimum near- and far-field detection beam geometries indicates that a practical mode-mismatched TL instrument should be based on the far-field detection geometry. The computation results further demonstrate that the optimum beam geometry and the TL amplitude depend largely on the excitation-beam profile. The top-hat beam TL experiment is approximately twice as sensitive as the Gaussian beam TL scheme.